Certain components, such as for example turbine blades and stator vanes for gas turbine engines, because of their relatively complex shapes and harsh operating environment to which these components are composed, are typically cast of nickel-based and cobalt-based alloys which are conventionally known as superalloys, which have high strength and typically very high melting temperatures.
The strength of such components is enhanced by forming the turbine vanes (stators), and particularly the turbine blades, using directional solidification casting for obtaining substantially single crystal components. Such a process is conventionally known.
Various processes and apparatus for directional solidification casting, such as directional solidification casting apparatus and method disclosed in U.S. Pat. Nos. 4,108,236 and 4,175,609, are known in the industry and vary in effectiveness. In these processes, a suitable ceramic mold is specifically configured for the particular component being cast, such as a gas turbine engine blade or vane. The mold is lowered into a heating chamber where it is preheated, and subsequently then filled with a desired superalloy in a superheated liquid melt condition. Thereafter, the bottom of the mold is then subjected to preferential cooling to commence the unidirectional solidification process necessary for single crystal formation, which travels upwardly through the mold.
Cooling of the mold may be accomplished in different manners. In one conventional process, a suitable liquid metal coolant, such as molten tin or aluminum, is contained in a bath below the mold, with the mold then being immersed into the cooling bath for effecting a substantially large temperature gradient in the melt for enhancing directional solidification.
In a typical directional solidification casting furnace, solid superalloy known as a charge is initially placed inside a melting crucible surrounded by a suitable heater, such as an induction heater, which melts the charge to form the liquid melt with suitable superheat. The mold is initially positioned inside a heating chamber within a furnace, which preheats the mold to a suitable elevated temperature. And, the furnace and mold are disposed above a liquid metal cooling bath. These components are typically disposed within a common pressure vessel or housing which makes up the furnace, which is typically evacuated, or filled with a suitable inert gas.
During the process, the melt is poured from the melting crucible into the preheated mold. The mold is then lowered, bottom end first, into the bath for immersion cooling thereof to directionally solidify the melt upwardly inside the mold. Upon completion of melt solidification inside the mold, the mold is removed upwardly from the bath, furnace, and housing. A new charge and mold are placed inside the housing and the process is repeated to cast additional parts.
It is known that in order to obtain unidirectional crystal growth vertically upward, there need be uniform high thermal gradient in the axial (vertical) direction, so that there is a horizontal liquid-solid interface within the mold, with such interface moving vertically upwards as the metal cools. Accordingly, the cooling must occur unidirectionally in the vertical (axial) direction. Heat loss or a thermal gradient in the radial direction (ie. radially outwards of the mold) is undesirable and has a detrimental effect on unidirectional crystal formation with exterior portions of the mold tend to cool prior to interior portions of the mold resulting in a non-planar liquid-solid interface which has a detrimental effect on unidirectional cooling and thus unidirectional crystal growth formation. Any space between the heating chamber for the mold and the level of the liquid metal bath means that a portion of the mold, particularly its exterior, upon being lowered out of the heating chamber for immersion into the bath, will lose heat through radiation quicker than the interior of the so-exposed mold, resulting in an undesirable radial thermal gradient.
Accordingly, to achieve a purely axial (vertical) thermal gradient for unidirectional cooling, it is known to make the liquid metal bath variably positionable so as to be able to position the liquid metal container, and in particular the level of the liquid metal bath, immediately beneath the heating chamber. In such manner there is no space between the heating chamber and the liquid metal bath, and the mold when lowered out of the heating chamber for immersion into the coolant will be immediately thrust into such coolant. Such a furnace, having a liquid metal bath which may be positionable directly under the heating member, is taught in published EPO application 0631832A1 filed Feb, 2, 1993 and assigned to one of the applicants herein, namely ALD Vacuum Technologies Gmbh. In circumstances where the level of the liquid metal coolant is positioned immediately beneath the heating chamber, upon immersion of the mold within the container containing the liquid metal coolant, the level of the liquid metal coolant is caused to rise due to liquid metal within such container being displaced by the immersed mold. Any rise in the level of such molten liquid coolant is highly undesirable, since it will, being in such proximity to the heating chamber, immediately enter the heating chamber, and likely cause extensive damage to such heating chamber and other problems, such as vaporization of liquid metal coolant. Such problems are very acute where the heating chamber employs induction or resistance heating coils, and the liquid metal contacts such heating coils. In such circumstances damage to the heating coils typically results, due to their temperature generally being significantly higher than that of the liquid metal coolant.
To overcome such problems, the prior art practice has been generally to fill the bath container right to the top with liquid metal coolant, so that any excess will not rise into the heating chamber but will instead spill over the upper lip of the bath container and fall onto the floor of the furnace. This has the disadvantage not only of mess and build-up of solidified coolant on the floor of the furnace, but also has the definite disadvantage that liquid metal coolant must always be added to the bath container after each casting in order to conduct the next casting. Otherwise if no additional liquid metal is added to make up for the quantity of liquid metal lost through displacement, if in the next casting operation the bath container is moved so that the level of the remaining liquid metal is immediately beneath the heating chamber, upon immersion the level of the liquid metal coolant, the liquid metal now having room to rise in the bath container since some metal coolant had previously been lost, will then rise and enter the heating chamber, with undesirable and detrimental results.
A known means to overcome the problem of spillage of the liquid metal to the floor of the furnace is to provide a bath container having a spillway which catches and contains liquid metal coolant overflow, as disclosed and depicted in aforementioned published EPO application 0631832A1, namely in FIG. 4 thereof. Unfortunately, although the liquid metal "spillover" is retained by such spillway, it need subsequently be re-added to the bath container prior to the next casting operation. Frequently, such liquid metal has become solidified, which means it needs to be re-heated, and such wastes time and heat energy. More importantly, however, re-adding the "spillover" for each casting operation makes casting of numerous parts in succession time-consuming and inefficient, increasing the cost of each individually cast article. Alternatively, if in order to compensate for spillover, the bath container is made deep enough so that no "spill-over" over the top lip thereof occurs upon immersion of the mold therein, the operator of the furnace is left with two alternatives, neither of which is desirable. The operator may choose to position the level of the molten liquid metal in the bath container immediately below the heating chamber, in which case upon the immersion of the mold therein the level will rise and enter the heating chamber, with the detrimental results mentioned previously. Alternatively, the operator may choose to position the level of liquid metal slightly below the heating chamber, so that only after full immersion of the mold will the level of the liquid metal rise to the lowermost extremities of the heating chamber. Unfortunately, this means that a "space" will initially exist between the coolant bath and the heating chamber, which will cause the non-directional cooling problems discussed earlier.
Accordingly, a need exists for a furnace apparatus for unidirection solidification of superalloys which overcomes the above problems of the prior art.